Many newbie questions regarding python

MRAB python at mrabarnett.plus.com
Fri Oct 8 01:46:41 CEST 2010


On 08/10/2010 00:10, Rogério Brito wrote:
> Hi there.
>
> I am used to some languages like C, but I am just a complete newbie with Python
> and, while writing some small snippets, I had encountered some problems, with
> which I would sincerely appreciate any help, since I appreciate this language to
> write my "running pseudocode in" and I am seriously thinking of using it to
> teach some algorithms classes.
>
> 1 - The first issue that I am having is that I don't seem to be able to, say,
> use something that would be common for people writing programs in C: defining a
> one-dimensional vector and only initializing it when needed.
>
> For instance, in C, I would write something like:
>
> int v[20];
> for (i = 0; i<  20; i++)
>      v[i] = 0;
>
> Note that I only define the vector v (and its size) at the beginning but
> initialize it latter during the code per-se.
>
> My first try to write it in Python was something like this:
>
> v = []
> for i in range(20):
>      v[i] = 0
>
> Unfortunately, this doesn't work, as I get an index out of bounds when trying to
> index the v list. Of course, the main difference between the two snippets is
> that, in C, I declared v to have 20 positions, while in python I initialized it
> to be the empty list and, indeed, it has an empty set of indexes.
>
> What is the Pythonic way of writing code like this? So far, I have found many
> alternatives and I would like to write code that others in the Python community
> would find natural to read. Some of the things that crossed my mind:
>
>      v = [0 for i in range(20)]
>
>      v = [0] * 20
>
>      v = []
>      for i in range(20): v.append(0)
>
> What should I prefer? Any other alternative?
>
> If possible, I would like to simply declare the list and fill it latter in my
> program, as lazily as possible (this happens notoriously when one is using a
> technique of programming called dynamic programming where initializing all
> positions of a table may take too much time in comparison to the filling of the
> array).
>
Python doesn't have declarations. The code:

     v = []

simply creates an empty list and binds the name 'v' to it. If you want 
to create a list containing 20 zeros then:

     v = [0] * 20

is the Pythonic way. Which one you do depends on the particular problem
you're working on; do whichever makes the most sense.
>
> 2 - If I declare a class with some member variables, is is strictly necessary
> for me to qualify those members in a method in that class? For instance, if I
> define:
>
> class C:
>      f = 1
>      def g(self):
>          return f
>
> I get an annoying message when I try to call the g method in an object of type
> C, telling me that there's no global symbol called f. If I make g return self.f
> instead, things work as expected, but the code loses some readability.
>
> Is there any way around this or is that simply "a matter of life"?
>
The name 'f' in that case is an attribute of the class itself. If you
want it to be an attribute of an instance of the class then do
something like this:

     class C:
         def __init__(self):
             self.f = 1
         def g(self):
             return self.f

You should write Python code in the Python idiom and not try to look
for the Python equivalent of a C idiom. In other words, don't try to
write a C program in Python! You might want to read "The Zen of
Python". Just type:

     help("this")

at the Python prompt.



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